GaN series material has been widely used in blue and ultraviolet optoelectronic devices since it was introduced to the development of MIS (Metal-Insulator-Semiconductor) diode in the early 1970s and p-n structure light-emitting diode (LED) from the 1990s. GaN series material is characterized by its wide direct band gap, thermal stability, and chemical stability. GaN bulk is not easily fabricated, and therefore a substrate of hetero-material, such as sapphire and silicon carbide, is inevitably used to grow GaN structure. Most of all, the sapphire substrate which is reported to have 14% lattice mismatch to GaN becomes the mainstream in the market.
Nevertheless, sapphire is commonly known as an electrical insulator and has poor thermal conductivity. To make a vertical type nitride light-emitting diode, or to improve the thermal dissipation performance of the light-emitting diode, a technology using an alternative carrier for replacing the sapphire also comes into practice. One of the separation methods called “laser lift-off” uses Nd-YAG or excimer laser to decompose GaN into gallium and nitrogen, and thus remove the sapphire substrate. However, it is not easy to control the laser beam penetration depth to stop just before the primary epitaxial structure so that the light-emitting quality is often deteriorated. The pertinent literature can be referred to U.S. Pat. No. 6,559,075 and No. 6,071,795.